Since a fluoropolymer has superior non-stick property, it has wide applications in various application goods which require non-stick property such as frying pan, a bread mold and a rice cooker. Although the fluoropolymer has superior non-stick property originated from surface free energy when compared with other resins, it has, on the other hand, difficulty of adhesion to other substances, and therefore it has a problem that the formation of a fluoropolymer coating film onto a substrate is not easy.
For example, a fluoropolymer such as polytetrafluoroethylene(PTFE), a tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer (PFA), tetrafluoroethylene-hexafluoropropylene (FEP) has been used in various applications because of being excellent in various characteristics such as water-repellent and oil-repellent properties, abrasion resistance, anti-fouling property, heat resistance and chemical resistance. However, it is generally expensive and there may be the case where mechanical strength or dimensional stability becomes insufficient. Accordingly, to cover such a disadvantageous point, while utilizing the advantages of the fluoropolymer, various trials have been performed to adhere the fluoropolymer coating film to other substrates such as metal, glass, ceramic and plastic.
When coating the fluoropolymer onto the surface of metal, ceramic, glass and plastic or the like, there has been a widely used method which comprises forming physical roughness on the surface of these substrates by a method of sandblasting, etching or the like, forming a primer layer by coating a primer composition having adhesive property to the surface of the substrate, and then adhering the fluoropolymer, which is a top coat layer, to the substrate through this primer layer. In this method, in adhesion between the substrate surface and the primer layer, anchor effect has been utilized where adhesive strength is enhanced by increasing adhesion area between the primer layer and the substrate surface owing to roughness of the substrate surface (refer to PATENT LITERATURE 1).
As a primer layer, a composition in which a mixture of a phosphoric acid and a chromic acid is added to a fluoropolymer, or a composition in which a polyethersulfone resin (PES), a polyamidimide resin (PAI), a polyimide resin (PI) or a polyphenylene sulfide (PPS), or the like is used as a main component, and a fluoropolymer is added thereto, is well known (refer to PATENT LITERATURE 2, 3, 4 and 5).
In applications where corrosion resistance is required, a fluoropolymer is generally required to have thickness, and therefore in order to increase the thickness, layered coating is necessary, where applying a powder coating material comprising the fluoropolymer and then baking at a temperature higher than melting point of the fluoropolymer are repeated. A primer is required to have heat resistance for enduring baking at high temperature for this long period of time, and maintaining adhesive property with the substrate or the like.
As the primer superior in heat resistant adhesive property, a chromium phosphate-based primer, which has superior resistance to baking at high temperature for a long period of time, has been used widely. However, with increase in consciousness to an environmental problem, development of a chromium-free primer that does not contain hexa-valent chromium and has strong heat resistant adhesive property comparable to a chromium phosphate-based primer has been strongly required for many years.
As a chromium-free primer, combinations of the fluoropolymer and various binder resins have been investigated. As the binder resin, use of polyphenylene sulfide (PPS) has been proposed, in view of heat resistance. However, PPS is inferior in compatibility with the fluoropolymer, and had a problem of insufficient adhesive property with a fluoropolymer layer.
In order to improve adhesive property with the fluoropolymer layer, as a binder resin in the chromium-free primer, addition of polyamidimide (PAI) and/or polyimide (PI) to PPS has been proposed (refer to PATENT LITERATURE 6 and 8), however, the chromium-free primer of these proposals had still insufficient heat resistant adhesive property.
In addition, there has also been proposed a chromium-free primer of an aqueous dispersed substance having PPS and PAI as a binder resin, which is capable of being applied to a smooth surface (refer to PATENT LITERATURE 7), however, this proposal is characterized by compounding two kinds of fluoropolymers with different melt viscosity in specific weight ratio, and had a problem of deterioration of heat resistant adhesive property, when baked for a long period of time.
Further, there has been proposed a primer coating composition composed of polyamidimide resin (PAI) and a polyphenylene sulfide resin (PPS) in 0.1 to 20% by mass of total amount (refer to PATENT LITERATURE 9), however, also in the primer of this proposal, further improvement of heat resistant adhesive property has been required.
In addition, such a primer layer has been proposed, that has a structure where the first primer layer containing the fluoropolymer and an organic titanate having strong inorganic nature, and the second layer containing the fluoropolymer and polyphenylene sulfide are sequentially layered (refer to PATENT LITERATURE 10).